The second-oldest HIV sample ever discovered has pushed back the date the virus jumped from chimps to humans more than 2 decades, according to new research. The study helps further clarify how the AIDS epidemic started, and it reinforces the mounting evidence that HIV did not spread readily until cities emerged in central Africa.

The sample comes from a lymph node biopsy taken in 1960 from a 28-year-old woman in what is now Kinshasa, Democratic Republic of the Congo. Researchers led by evolutionary biologist Michael Worobey of the University of Arizona, Tucson, analyzed tissue blocks taken from 27 patients between 1958 and 1960 that the University of Kinshasa had kept along with some patient records. As Worobey notes, these patients had biopsies taken because they had mysterious ailments, including lymph node abnormalities, which are common in HIV-infected people. "You really couldn't ask for a better cohort to look for people who had HIV in Kinshasa," he says.

As researchers describe in tomorrow's issue of Nature, they isolated about 5% of HIV's genome from the woman's lymph. A comparison to the oldest HIV sample, which is also from Kinshasa and dates to 1959, found that they differed by about 12% in one overlapping region. This led Worobey and colleagues to estimate that the two viruses had a common ancestor around 1908, which means that the HIV that sparked the current AIDS epidemic jumped from chimpanzees to humans before then. Previous estimates dated the common ancestor to about 1931.

Worobey and co-authors link the spread of HIV to the growth of Kinshasa and other cities in central Africa. They document that no city in the region had more than 10,000 residents until 1910, backing the long-held theory that HIV needed this critical mass of people in an urban setting to establish itself. "The virus really struggles to ignite an epidemic" until cities permitted rapid transmission, notes Worobey. He says this underscores that concerted prevention efforts today can stop local epidemics from gaining a foothold.

Worobey, whose earlier studies[2] detailed how HIV spread from Africa to the United States and Europe, still has hundreds of samples to analyze and is searching for even older samples. "People just haven't really tried to find these old samples," he says.

Beatrice Hahn, whose group at the University of Alabama, Birmingham, inferred the previous estimates using modern HIV samples, applauds the hard work. "This was done just the way you want to do it," says Hahn, who notes that a second lab independently confirmed the sequence, which never happened with the 1959 sample. "This is like bending over backwards to do it the right way."